Emulating an Anechoic Environment in a Wave-Diffusive Medium Through an Extended Time-Reversal Approach

Abstract

A generalized time-reversal (TR) technique for the generation of coherent wavefronts within complex media is presented in this paper. Although completely general, this method is primarily considered for testing purposes herein, where an equipment under test is submitted to a series of impinging wavefronts with varying features. Electromagnetic compatibility, antenna testing as well as telecommunications facilities where complex-wavefront schemes (e.g., multi-path configurations) are required, could benefit from the proposed approach. The main advantages and limitations of current standard TR approaches are reviewed in this respect, exposing their inadequacy for this particular context. The proposed alternative technique, named time-reversal electromagnetic chamber (TREC) is introduced and studied by means of a formal theoretical analysis, showing how a reverberation chamber (RC) supporting a diffused-field condition can be operated as a generator of deterministic pulsed wavefronts.The TREC is demonstrated to be capable of generating arbitrary wavefronts with a remarkable accuracy, allowing to revisit the RC as a deterministic facility: the main advantages of RCs and anechoic ones are merged, leading to a new facility capable of potentially generating in real-time pulsed wavefronts while using low input energies, without requiring neither mechanical displacements nor any special features of the sources.

Steering Focusing Waves in a Reverberation Chamber With Generalized Time Reversal

“…Generalized TR (GTR) was first tested in an embryonic version in [7], [8], where direct wavefront synthesis was not yet formalized, as later done from a theoretical point of view in [9]. It is based on the use of the equivalence theorem as a way of avoiding the introduction of a source, in order to directly synthesize signals capable of generating focusing Part of the work here reported was funded by the French National Research Agency through the grant ANR-12-ASTR-0005, MIMOCHIC project, within the framework of the 2012 ASTRID program.…”

“…The simplest way to understand it is by going through the different steps at its origin. The interested reader can refer to [9] for a much more detailed discussion.…”

“…In the second step, y(t) is computed from an assumed knowledge of the Green's functions on the medium, acting as transfer functions between the equivalent sources over Σ and the receiving antenna, hereafter referred to as N e (r, ω) and N m (r, ω), respectively for the electric and magnetic field Green's functions. The result is a direct synthesis formula for the Fourier spectrum of the received signal y(t) [9]…”

“…Having verified the accurate reproduction of the convergent wavefront, it is fundamental to assess whether GTR can generate it with the same accuracy even when its focus and directions of arrival are changed. The theoretical derivations in [9] concluded that the homogeneous characteristics of diffusive media should ensure a comparable reproduction for any set of parameters, at least on average.…”

Multiple-Source Time-Reversal Transmissions in Random Media

“…As a matter of fact, single-source configurations have been demonstrated to be capable of recreating high-fidelity versions of a wavefront, when operated by time-reversed signals in a complex medium [20], [22]- [26]. In this case the use of more than one emitter is no longer justified on the basis of the equivalence theorem, but rather on the need to mitigate the level of these fluctuations, by averaging them out thanks to spatial/angle/polarization diversity.…”